1. Field of the Invention
The present invention relates to a method for performing roaming in a dual mode mobile terminal, and more particularly to a method for performing roaming in a dual mode mobile terminal in such a manner that a ping-pong phenomenon, occurring under a weak electric field between communication networks providing communication services using different mobile communication techniques, can be prevented.
2. Description of the Related Art
Currently, a Code Division Multiple Access (CDMA) system has been already distributed over a wide area with a sufficient infrastructure. However, a third generation network such as a Wideband Code Division Multiple Access (WCDMA) network has been gradually introduced in order to provide services which cannot be supported by a second generation network such as the CDMA network. In addition, a dual mode mobile terminal designed to receive a service specialized for the third generation network in an area located in the third generation network while using the infrastructure of the second generation network has been developed.
Lately, a dual mode user equipment or a dual mode mobile terminal simultaneously supporting the WCDMA and the CDMA has been developed to satisfy the need for providing a user with a mobile terminal capable of operating using both techniques (WCDMA and CDMA). One dual mode mobile terminal utilizing the two modes requires various techniques capable of most suitably operating in two modes. Among these techniques, roaming is performed when the terminal internally switches between the WCDMA mode and the CDMA mode.
Generally, the roaming function enables a dual mode mobile terminal to perform switching between the CDMA mode and the WCDMA mode, so that the dual mode mobile terminal operating in the WCDMA mode in a WCDMA service area can switch the WCDMA mode into the CDMA mode when the dual mode mobile terminal leaves the WCDMA service area.
The roaming function in an area in which the CDMA system and the WCDMA system are installed will be described with reference to FIG. 1. FIG. 1 illustrates an area in which both a typical CDMA system and a typical WCDMA system are installed. The area, in which both a typical CDMA system and a typical WCDMA system are installed, is classified into a single CDMA area supporting only a CMDA service and a superposition area simultaneously supporting a WCDMA service as well as the CDMA service.
When a dual mode terminal 10, initially located within a WCDMA network (i.e., in the superposition area 30) and operating in the WCDMA mode leaves the WCDMA network and travels into a single CDMA network 40, the dual mode terminal 10 searches for a CDMA base station because a WCDMA base station is not available within the single CDMA network 40. Then, if the dual mode mobile terminal 10 finds the CDMA base station, the dual mode mobile terminal 10 switches from the WCDMA mode into the CDMA mode. In contrast, if the dual mode mobile terminal 10 goes into the superposition area 30 from the single CDMA area 40, the dual mode mobile terminal 10 in the CDMA mode performs the roaming function in order to switch the CDMA mode into the WCDMA mode according to a predetermined condition.
Generally, if the dual mode mobile terminal 10 operating in the WCDMA mode leaves the WCDMA service area, the dual mode mobile terminal 10 performs the roaming function to switch to the CDMA mode. Similarly, the dual mode mobile terminal 10 can determine based on system parameters received from the CDMA base station if the dual mode mobile terminal 10 is positioned at the superposition area capable of providing a WCDMA service or at an area supporting only a CDMA service. The determination may be achieved even when the dual mode mobile terminal 10 is in the CDMA mode. Accordingly, if it is determined that the dual mode mobile terminal 10 is positioned at the WCDMA superposition area 30 based on the system parameters received from the CDMA base station, the dual mode mobile terminal 10 operating in the CDMA mode performs roaming for the WCDMA mode with a predetermined time interval.
When, as shown in FIG. 1, the dual mode mobile terminal 10 moves to a position at a border 20 adjacent to the CDMA network in the WCDMA area, the dual mode mobile terminal 10 receives system parameters from the WCDMA base station in a border between the superposition area 30 and the single CDMA area 40 and performs a handover to the CDMA network 40. In other words, the dual mode mobile terminal 10 communicates with the CDMA network. However, if it is determined that signal intensity of the CDMA network in a shadow area indicated by reference numeral 50 results from a weak electric field and has at most a preset threshold value, the dual mode mobile terminal 10 performs a handover to the WCDMA network 30. However, if signal intensity received from the WCDMA network 30 has at most the preset threshold value, the dual mode mobile terminal 10 performs handover by performing the roaming toward the CDMA network 40 again. Accordingly, as the dual mode mobile terminal passes through a shadow area 50, which is an area having weak CDMA receive signal intensity, a ping-pong phenomenon occurs in which a handover procedure is repeated by continuously performing the roaming function from the WCDMA network 30 to the CDMA network 40. A WCDMA weak electric field area also frequently has a ping-pong phenomenon in which the CDMA network controls the dual mode mobile terminal 10 again if the dual mode mobile terminal 10 does not perform handover toward the WCDMA network even though the dual mode mobile terminal 10 operating in the CDMA mode attempts roaming toward the WCDMA network with a predetermined time interval.
As described above for the conventional system, when a dual mode mobile terminal attempts roaming from one mode to another mode in a weak electric field area, if the one mode is not switched into another mode, the mode of the dual mode mobile terminal returns to the original mode, called a “ping-pong phenomenon”.
As described above, when roaming is repeatedly performed between different networks according to the intensity of a received signal, a probability an erroneous operation of the dual mode mobile terminal may increase. In addition, when full-searching is performed to search the cells in the WCDMA, the dual mode mobile terminal remains in a system switching state during the range of 20 seconds to a maximum of 2 minutes. Accordingly, the dual mode mobile terminal does not receive services during the system switching state, thereby causing inconvenience for a user receiving the communication service.